Process for foaming a sheet of ethylenic resin during downward movement of the sheet

ABSTRACT

Process and apparatus for producing a wrinkle-free, highly foamed sheet of an ethylenic resin having a uniform and fine cellular structure by causing a long strip of a sheet-like molded article of a cross-linked ethylenic resin containing a normally solid organic blowing agent to fall downwardly and continuously transferring it in the falling direction, heating the sheet-like molded article in transit to a foaming temperature, the heating being controlled so that a starting position of foaming along the widthwise direction of the sheet-like article will not appreciably fluctuate upwards or downwards and extending the foamed sheet-like article in the widthwise direction.

United States Patent mr Sagane et a1. 1 1 Jan. 16, 1973 1 PROCESS FORFOAMING A SHEET OF 3,250,731 5/1966 Buhl ..264/54 ux ETHYLENIC RESINDURING 3,387,328 6/1968 Winstead ..264/51 UX WNW MOVEMENT OF THE 3,426,11 1 2/1949 Simpson ..264/51 X SHEET 3,452,123 6/1969 Beckmannm,.....264/54 X 3,470,l l9 9/1969 Benning ..264/54 X [75] Inventors: NorioSagane, Kyto; Shinsaku Nakata, Toyonaka-shi; Hiroshi OTHER PUBLICATIONSUeda; UraWa-Shi; Teiji Matsumara, Modern Plastics Encyclopedia Issue for1963, V01. 40, Minamisaitama-gun, all of Japan 1 Sept 19 2 pp 7 4 [73]Assignee: Sekisui Kagaku Kogyo Kaubushiki Kaisha, Osaka Japan PrimaryExaminer-Donald J. Arnold Assistant ExaminerPau1 A. Leipold Filed: 1-1969 Att0meySherman and Shalloway [21] Appl. No.: 861,288

[57] ABSTRACT [52] US. Cl. 264/54, 18/45, 18/5 P, Process and apparatusfor Producing a Wrinkle-free 18/15 R, 264/93, 2 4 209 2 4 2 0 264/288highly foamed sheet of an ethylenic resin having a 51 Int. Cl. ..B29d27/00 uniform and fine Cellular Structure y causing a long 8 Field fSearch; "264/54, 51, 289, 210 2 g strip of a sheet-like molded articleof a cross-linked ethylenic resin containing a normally solid organic 5k f cit d blowing agent to fall downwardly and continuously transferringit in the falling direction, heating the UNITED STATES PATENTSsheet-like molded article in transit to a foaming tem- 3,562,367 2/1971Shinohara ..264/54 x heathg being comroued that a Start 3.055048 9/l962Koppeheleum 'ing position of foaming along the widthwise direction3,098,831 7/1963 Carr ..264/54 x of the sheet-like article will not pp yfluctuate 3,108,851 10/1963 Hofer ..264/289 X upwards or downwards andextending the foamed 3,1 18,161 1/1964 Cramton 1 ..264/54 X sheet-likearticle in the widthwise direction,

3,229,005 1/1966 Reifenhauser ..264/210 X 1/1966 Cheney ..264/210 X 10Claims, 6 Drawing Figures III/III.

PATENTED JAN 1 6 I973 SHEET 1 OF 4 IIIIIII/I/IIII/I/ IUIIIII/IIIIIIIIllllll lf PROCESS FOR FOAMING A SHEET OF ETHYLENIC RESIN DURING DOWNWARDMOVEMENT OF THE SHEET This invention relates to a process for theproduction of a foamed sheet of an ethylenic resin and to an apparatusfor use in such process. More particularly, it relates to a process forthe production of a foamed sheet of an ethylenic resin, which comprisesheating a flat or tubular sheet-like molded article of a cross-linkedethylenic resin containing a normally solid organic blowing agent tothereby foam the article, and to an apparatus for practicing suchprocess.

It has heretofore been known to produce a foamed polyethylene sheet byapplying ionizing radiation such as accelerated electrons to apolyethylene sheet containing a normally solid organic blowing agent orsubjecting it to a cross-linking treatment with a cross-linking agentsuch as organic peroxides and heating the cross-linked polyethyene sheetto effect its foaming. Such cross-linking treatment causes thevisco-elasticity of the polyethylene sheet during heating to be wellbalanced with its expansion by gases liberated on decomposition of theorganic blowing agent, with the result that the polyethylene sheet iswell foamed and not shrunken afterwards. Thus, a polyethylene sheetfoamed to a high ratio can be obtained.

In the above-mentioned conventional manufacture of foamed sheets, thefollowing three methods are mainly employed to foam the sheets byheating. One of the methods comprises placing an unfoamed sheet in aclosed cavity mold, heating it and removing the foamed sheet by openingthe mold. Because of the use of a mold, this method must be practicedbatchwise, and the productive efficiencies become low. Otherdisadvantages are that the method cannot give a long strip of foamedsheet and the surface of the foamed sheet becomes roughened.

Another method of heat-foaming comprises heating an unfoamed sheet whiletransferring it on a belt conveyor. This method is advantageous in thata long strip of sheet can be obtained continuously. But it has thedisadvantage that during the expansion of an unfoamed sheet, the sheetbecomes tacky and adheres to the belt conveyor, and this prevents a freeexpansion of the sheet and results in foamed sheet products having anon-uniform thickness and many wrinkles. Furthermore, the surfacecondition of the obtained foamed product differs between top and bottom,and the commercial value of the products become lower.

The last of the three methods comprises supporting an unfoamed sheetafloat on a bath of liquid such as ethylene glycol heated to the foamingtemperature and heating it from above by means of infrared rays and thelike. This method is applicable where it is desired to obtain a longstrip of foamed sheet continuously. Considerable difficulties are,however, encountered in transferring an unfoamed sheet afloat a liquidbath. A free three-dimensional expansion is incomplete, and theresulting foamed products have a non-uniform thickness and many wrinkleswith the state of the surface being different between top and bottom.Also, there is a tendency that the foamed sheetis discolored by thedeposition of the liquid, and therefore, it becomes necessary to providea step of removing the liquid deposited on the foamed sheet.

In addition to these various defects of the conventional heat-foamingmethods, the foamed sheets obtained by these methods have a non-uniformcellular structure in which large cells are present together with smallones; therefore, the foamed sheets have deteriorated physical propertiessuch as heat-insulating properties and elasticity, and their commercialvalues are lowered.

One object of the present invention is to produce a foamed sheet of anethylenic resin, which has a considerably uniform thickness and a smoothsurface and is free from wrinkles.

Another object of the invention is to provide a foamed sheet of anethylenic resin, which is foamed to a highratio and has a very uniformand fine cellular structure.

Still another object of the invention is to produce a long strip of afoamed sheet of an ethylenic resin efficiently and continuously on anindustrial scale.

Many other objects and advantages of the invention will become apparentfrom the following description.

The above-mentioned objects of the present invention can be achieved bycausing a long strip of a sheetlike molded article of a cross-linkedethylenic resin containing a normally solid organic blowing agentcapable of liberating gases upon decomposition by heating to falldownwardly and continuously transferring it in the falling direction,heating the sheet-like molded article in transit to a temperature abovethe softening temperature of the cross-linked ethylenic resin and abovethe decomposition temperature of the organic blowing agent to therebyfoam the sheet-like molded article, the heating being controlled so thata starting position of foaming along the widthwise direction of thesheet-like article will not appreciably fluctuate upwards or downwards,and extending the foamed sheet-like article in the widthwise directionby a ratio corresponding to expansion of the foamed sheet-like articlein its widthwise direction to thereby remove wavy wrinkles occurring inthe widthwise direction of the article at the time of foaming.

The most essential feature of the present invention is to cause thesheet-like molded article to fall downwardly and control the heating ofthe sheet-like molded article so that a starting position of foamingalong the widthwise direction of the sheet-like article will notappreciably fluctuate upwards or downwards.

When a sheet-like molded article of an ethylenic resin is heat-foamedwhile it is falling downwardly, both surfaces of the sheet-like articleare uniformly heated and foamed in an atmosphere of hot air withoutcontacting the supporting device. Consequently, a threedimensional freeexpansion of the sheet-like article is not hindered at all, and thesurface of the foamed sheet-like article becomes smooth and beautiful byexposure to the hot air atmosphere. Furthermore, when the sheet-likematerial is heat-foamed while falling downwardly, the starting positionof foaming which is situated along the widthwise direction of thesheet-like article fluctuates greatly upwards and downwards to causenon-uniformity in the thickness of the obtained foamed sheet-likearticle. If this position is rendered stationary as much as possible bycontrolling the heating, the resulting foamed sheet-like article comesto have a considerably uniform thickness.

Experiments have confirmed that if the starting position of foamingfluctuates within the distance of 30 cm upwards and downwards, thefluctuation in thickness of the obtained foamed sheet-like article iswithin about :20 percent, and no problem arises in the use of the foamedsheet-like article for practical purposes. It has also been confirmedthat if the heating temperature is controlled considerably accurately,the starting position of foaming can be fixed without any fluctuationperceptible with the naked eye, and accordingly, the thickness of thefoamed sheet-like article can be made remarkably uniform.

Generally, when a sheet-like molded article of an ethylenic resin isheat-foamed while it is falling downwardly, it is likely to be elongatedexceedingly and even to breakage. It has, however, found that if thesheet-like article is appropriately cross-linked, such excessiveelongation or break does not occur, and the operation becomes easy.

We are the first to discover that when a sheet-like molded article of across-linked ethylenic resin containing an organic blowing agent isheat-foamed while it is falling downwardly, and at this time, theheating is controlled so that a starting position of foaming along thewidthwise direction of the sheet-like article will not fluctuate upwardsand downwards, there is obtained a foamed sheet-like molded articlehaving a uniform thickness.

The heating temperature is preferably controlled by the provision of apre-heating chamber and a foaming chamber each of which is equipped withradiation heaters such as infrared heaters. Hot air is introduced intothe pre-heating chamber and the foaming chamber independently from eachother. The sheet-like article is pre-heated in the pre-heating chamberto a temperature lower than the foaming temperature, and then thearticle falling vertically in the heating chamber is rapidly heated tothe desired foaming temperature. The heating is further accuratelycontrolled by the radiation heater such as infrared heaters to avoid theup-anddown fluctuation of the starting position as much as possible.

The so obtained foamed sheet-like molded article has wrinkles caused byexpansion along the widthwise direction thereof. These wrinkles areremoved in accordance with the invention by extending the width of thesheet-like article by a ratio corresponding to expansion in thewidthwise direction. For effecting this widthwise extension, a clothguider or tenter may be used when the foamed article is a flat sheet.But the side portions of the foamed sheet held by the cloth guider ortenter tend to be marred, and therefore must be trimmed. For thisreason, it is preferable to use a pneumatic system utilizing a suctionforce of air in this extending process. The pneumatic system will bedescribed later in the specification. If the obtained article is atubular sheet, it can be extended by pressure of air introduced into thetubular sheet. But this method has the defect that it is not easy tokeep the foamed tubular sheet dimensionally stable. It is preferabletherefore to use a pneumatic system utilizing a mandrel which will bedescribed later in the specification. It is to be noted that we are alsothe first to use such pneumatic systems for the above-mentioned purpose,and this is one of the essential features of the present invention.

When a foamed sheet-like molded article is extended in a widthwisedirection in the present invention, it is transferred while containingroughly uniform wavy wrinkles, because the up-and-down fluctuations ofthe foaming start position along the widthwise direction of thesheet-like article are minimized as much as possible. Consequently, itis possible to effect the extending operation always under predeterminedconditions, and to remove the widthwise, wavy wrinkles completely.

The foamed sheet which has been deprived of the wavy wrinkles by suchextending operation is cooled by allowing it to stand at the temperatureof the atmosphere, or is positively cooled, then taken up by a take-updevice, and is generally wound up in a roll form. At this time, thetake-up rate of the take-up device is generally the feed rate of theunfoamed sheetlike molded article plus a rate corresponding to theexpansion of the sheet-like article in its longitudinal direction, or isslightly higher than this.

The cross-linked ethylenic resins containing the normally solid organicblowing agent capable of liberating gases on decomposition by heatingare produced by the following two methods.

One of the methods comprises adding to an ethylenic resin an organicblowing agent having a decomposition temperature higher than the meltingtemperature of the resin, mixing them uniformly by means of such adevice as ribbon blender and Banbury mixer, melting and kneading theresulting mixture by an extruder or calender roll at a temperature atwhich the organic blowing agent is not substantially decomposed tothereby shape the mixture into a sheet form, and thereafter subjectingthe sheet-like article to an ionizing radiation whereby the ethylenicresin that constitutes the sheet-like article is cross-linked. The othermethod comprises adding to an ethylenic resin an organic blowing agenthaving a decomposition temperature higher than the melting temperatureof the resin and an organic peroxide having a decomposition temperaturelower than the decomposition temperature of the organic blowing agentand a decomposition temperature higher than the melting temperature ofsaid resin, mixing them uniformly by a ribbon blender or a Banburymixer, melting and kneading the resulting mixture by an extruder or acalender roll at a temperature at which the organic peroxide is notsubstantially decomposed to thereby shape the mixture into a sheet form,and heating the sheet-like article to a temperature at which the organicperoxide is substantially decomposed and at which the organic blowingagent is not substantially decomposed whereby the ethylenic resin thatconstitutes the sheet-like article is crosslinked.

The ethylenic resin used in this specification includes homopolymers ofethylene such as low-density polyethylenes, medium-density polyethylenesand highdensity polyethylenes and copolymers of ethylene such as anethylene/vinyl acetate copolymer, an ethylene/propylene copolymer, anethylene/butadiene copolymer, an ethylene/butene-l copolymer and,ethylene/vinyl chloride copolymer. Preferable ethylenic resins have anaverage molecular weight of 10,000 to 100,000.

The kneading and melting temperature is slightly higher than the meltingpoints of the ethylenic resins, and is usually to C. At thesetemperatures,

the organic blowing agents and organic peroxides used in-the presentinvention are not substantially decomposed.

The ethylenic resins used in the present invention may contain a rubberysubstance such as natural rubber, butadiene rubber, isobutylene rubber,an acrylonitrile/butadiene copolymer rubber and a styrene/butadienecopolymer rubber. When such a rubber substance is incorporated into theethylenic resins, the tensile strength and elasticity of the resultingfoamed sheet of the ethylenic resin are improved.

In view of the melting temperatures of the ethylenic resins,azodicarbonamide (decomposition temperature about 190 C.),N,N-dinitrosopentamethylene tetramine (decomposition temperature about204 C.) and p,p'-oxybis(benzenesulfonyl hydrazide) (decompositiontemperature about 164 C.) are employed with good results as the normallysolid organic blowing agents which liberate gases on decomposition byheating. The amounts of these blowing agents are determined dependingupon a ratio of foaming of the intended foamed sheet. Roughly, theamounts of these blowing agents are one-half of the foaming ratio(expressed in parts by weight) per 100 parts by weight of the ethylenicresin. For example, if it is desired to obtain a foaming ratio of 30,the organic blowing agent is added in an amount of about 15 parts byweight per 100 parts by weight of the ethylenic resin. Among theseorganic blowing agents, azodicarbonamide is most preferable because hisrapidly decomposed without fear of coloration and explosion and moreoverthe kneading and melting temperatures of an ethylenic resin containingthis blowing agent are well balanced with the foaming temperaturethereof, and a foamed sheet with very fine cells is obtained.

When azodicarbonamide as the organic blowing agent and a metal salt of afatty acid such as zinc stearate, calcium stearate, barium stearate,magnesium stearate, aluminum monostearate, aluminum distearate, aluminumpalmitate and aluminum octoate are added to a powdery low-densitypolyethylene, the azodicarbonamide is well dispersed uniformlythroughout the low-density polyethylene and is decomposed rapidly. As aresult, a better foamed sheet is obtained from this combination.Preferably, the amounts of the fatty acid metal salts are 0.5 to 3 partsby weight based on 100 parts by weight of the low-density polyethylene.

The ionizing radiation rays to be used in cross-linking the ethylenicresins are ,8 rays, 'y rays, neutron and electron. When a sheet-likemolded article of an ethylenic resin is subjected to ionizing radiation,a cross-linkage occurs among the molecules of the ethylenic resin. Thisbrings about a rise in the softening temperature of the resin, and makesits viscosity properties suitable for expansion thereof by heating.

If the dose of these ionizing radiation is less than 0.5 Mrad, theobtained multicellular product has low ratio of foaming and nonuniformcells. Moreover, when the unfoamed sheet-like material is transferred byfalling at the time of heat-foaming, the sheet-like material isremarkably elongated because of its own weight, thus making itimpossible to foam the sheet-like material well. If, on the other hand,the dose is above 20 Mrad, the degree of cross-linkage becomes excessiveand the sheet-like material is not well foamed. For this reason, thedose of the ionizing radiation is preferably in the range of 0.5 to 20Mrad. In the present invention, the use of electron rays as the ionizingradiation is preferable from the industrial viewpoint. For obtaining auniform irradiation, it is preferable to irradiate electron rays of thesame dose onto the top and the bottom surfaces of the sheet-like moldedarticle.

When the organic peroxides are used to cross-link the ethylenic resins,such compounds as dicumyl peroxide (decomposition temperature about C.),2,5- dimethyl-2,5-di(tertiary-peroxy)-hexane (decomposition temperatureabout 157 C.) and di-tertiary-butylperterephthalate (decompositiontemperature about 144 C.) are used with good results. Desirable organicperoxides have a decomposition temperature at least about 20 C. lowerthan the decomposition temperature of the organic blowing agent. Bestresults are obtained with a combination of azodicarbonamide as theorganic blowing agent and dicumyl peroxide as the organic peroxide.

Heat decomposition of the organic peroxides yields free radicals, andresults in the formation of a crosslinkage among the molecules of theethylenic resin that makes up the sheet-like molded article. Preferably,the amounts of the organic peroxides are such that the gel fraction ofthe cross-linked ethylenic resin in hot xylene is 30 45 percent byweight.

If a mixture of the ethylenic resin and'the organic blowing agent or amixture of the ethylenic resin, the organic blowing agent and theorganic peroxide is kneaded and melted and shaped into a sheet formafter removing the air present inside the mixture as much as possible,the cells of the resulting foamed sheet become exceedingly uniform andfine. When a sheet-like molded article containing air is foamed byheating, the obtained foamed sheet contains both large cells caused byexpansion of the air and smaller cells caused by the decomposition gasesof the organic blowing agent. if,

however, the sheet-like molded article is heat-foamed after removing airpresent therein as much as possible, there is hardly any influence ofthe air, and a foamed sheet having exceedingly uniform and fine cellscan be obtained.

Again, we are the first to apply to the manufacture of a foamed sheet,with good results, a technique of kneading and extruding such a mixtureafter having removed the air present therein as much as possible, andthis constitutes another essential feature of the present invention.

For realizing this, usually employed are a method comprising feeding themixture into a vacuum hopper provided at a feed stock inlet of ascrew-type extruder to suck and remove the air inside the mixture, andextruding it into a sheet form while kneading and melting it in theextruder, and a method comprising sucking and removing the air presentin the mixture from a vent provided in an extruder of the screw type andconcurrently kneading and melting the mixture and extruding it into asheet form. The former method is particularly preferred in respect ofoperation and effect.

Extruders provided with a vacuum hopper or a vent are obvious to thoseskilled in the art, and do not appear to need description.

Our experiments have indicated that when a mixture consisting of 100parts by weight of a powdered polyethylene having a melt index of 4.0and a specific gravity of 0.922 (passable through a IO-mesh screen) and10 parts by weight of azodicarbonamide is kneaded and melted at 135 C.by an extruder without removing the air present inthe mixture and thenextruded into a sheet form, a sheet having a specific gravity of 0.82 isobtained; and that when the obtained sheet is subjected to theirradiation of 3 Mrad electron rays and heatfoamed at 200 C., it isexpanded to about 20 times and there is obtained a foamed sheet havingcells with a diameter of about 0.5 mm. On the other hand, we have foundthat when an extruder provided with a vacuum hopper is used and afterremoving the air inside the mixture by reducing the pressure of thevacuum hopper to 350 mmI-lg, 450 mmHg and 550 mmHg respectively below heatmospheric pressure to discharge air from the mixture, the mixture iskneaded and melted at 135 C. by means of the extruder and then shaped,there is obtained a sheet having a specific gravity of 0.86, 0.90 and0.93 respectively; and that when this sheet is subjected to theirradiation of electron rays of 3 Mrad and heated to 200 C. inaccordance with the process of the present invention, it is expanded toabout 20 times and there is obtained a sheet having cells with adiameter of about 0.3 mm, 0.2 mm and 0.1 mm respectively.

It is clear from these experiments that if air inside the mixture isdischarged as much as possible, it is possible to reduce the sizes ofcells of the obtained foamed sheet to a remarkable degree.

Specific embodiments of this invention have been chosen for purposes ofillustration and description, and are shown in the accompanyingdrawings, forming a part of the specification, wherein:

FIG. 1 is a schematic view of the apparatus for producing the sheet ofthe invention;

FIG. 2 is a schematic view of the apparatus for irradiating acceleratedelectrons to the sheet in accordance with the invention;

FIG. 3 is a sectional view, partly broken away, of an apparatus forfoaming a flat sheet;

FIG. 4 is an enlarged side elevation, partly broken away, of theextended part of the sheet shown in FIG. 3;

FIG. 5 is a partly-broken-away, enlarged sectional view taken along theline IV-IV of FIG. 4; and,

FIG. 6 is a perspective view, partly broken away, of an apparatus forfoaming a tubular sheet.

Referring to FIG. 1, a screw type extruder, shown at 50, is providedwith a vacuum hopper 52 at its stock feed inlet 51. The vacuum hopper 52is divided into a first chamber 53 and a second chamber 54, with a valve55 used for partitioning. The valve 55 is moved up and down by means ofan air cylinder 56 for closing and opening of the two chambers 53 and54. The vacuum hopper 52 also includes a material throw-in opening 57, asuction pipe 58 for reducing the pressure of the first chamber 53, and asuction pipe 59 for reducing the pressure of the second chamber 54. AT-die 60 is fitted to the top of the extruder 50. Reference numeral 61shows a take-up device consisting of a pair of pinch rolls, and 62, awind-up device.

Referring to FIG. 2, the reference numeral 63 indicates a chamber forirradiating accelerated electrons, which includes accelerated electrongenerators 64 and 65. Reference numeral 66 indicates a guide roll, 67, atake-up device consisting of a pair of pinch rolls, 68, a wind-updevice, and 69, a delivering device.

In FIG. 3, a pre-heating chamber is shown at 2, and a foaming chamberconnected to said pre-heating chamber 2, at 3. Both the pre-heatingchamber 2 and the foaming chamber 3 are constructed of heat-insulatingwalls. An upper wall of the pre-heating chamber 2, the boundary wallbetween the pre-heating chamber 2 and the foaming chamber 3, and a lowerwall of the foaming chamber 3 contain an aperture 4, 5, 6 respectivelyfor passage of a sheet. Above the aperture 4 of the pre-heating chamber2 is located a supporting device 7 for the sheet. The supporting device7 consists 1 of a pair of pinch rolls for delivery of the sheet so thatthe sheet is transferred into the preheating chamber 2 while beingsupported by this pair of pinch rolls. A number of infrared heaters 8and 9 are disposed in the pre-heating chamber 2 and the foaming chamber3, in parallel with the widthwise direction of the sheet and at certainintervals on both sides of the sheet in a symmetrical manner. Hot airinlets l0 and 11 are provided at the lower portions respectively of thepre-heating chamber 2 and the foaming chamber 3, and hot air outlets 12and 13 are provided at the upper portions respectively of thepre-heating chamber 2 and the foaming chamber 3. Hot air introduced intothe preheating chamber 2 and the foaming chamber 3 from the hot airinlets 10 and l 1 respectively rises along both surfaces of the sheetand is exhausted outwards from the hot air outlets 12 and 13.

By partitioning the pre-heating chamber 2 from the foaming chamber 3 bythe boundary wall, the temperature of the pre-heating chamber 2 and thefoaming chamber 3 can be controlled independently.

The reference numeral 14 indicates a sheet extending device, 15, a groupof cooling rolls, and 16, a takeup device consisting of a pair of sheettake-up rolls. This pair of pinch rolls 16 are adapted to be rotated atthe feed rate of the unfoamed sheet-like molded article plus a ratecorresponding to the expansion of the sheetlike article in itslongitudinal direction, or at a rate slightly higher than this.

The details of the sheet extending device 14 is shown in FIGS. 4 and 5.In FIGS. 4 and 5, the reference numerals 14 and 14' indicate twoextending devices of the same construction. These are opposed to eachother in a symmetrical fashion by a distance corresponding to a width ofthe expanded sheet. The. reference numerals l7 and 17 indicate suctiondevices, and the sheet guide grooves 18 and 18' are each providedbetween facing surfaces of each suction device 17 or 17. The bottom ofeach of said guide grooves 18 and 18' has a number of suctionholes l9and 19' which lead respectively to pressure reducing chambers 20 and 20'respectively provided inside the suction devices 17 and 17'. The suctionholes 19 and 19 are designed to suck the outer air with 'vigor bydischarging air inside the pressure reducing chambers 20 and 20 fromexhaust pipes 21 and 21' by means of a vacuum pump (not shown).

The gap of each of said guide grooves 18 and 18' is determined accordingto the thickness of a sheet to be tentered, but generally about 1.5 tothree times the thickness of the sheet.

Moving belts 22 and 22' having a number of air vents are provided on thebottom surfaces of the sheet guide grooves 18 and 18 so that they canslide over the bottom surfaces in intimate contact. The moving belts 22and 22 are mounted on drive rollers 23 and 23 and guide rollers 24 and24, and are adapted to be moved by the drive rollers 23 and 23' at thesame velocity as the sheet take-up velocity. The moving belts 22 and 22'may be a cloth or a rubber belt provided with a number of smallapertures.

An embodiment of foaming a flat sheet by means of the apparatus shown inFIGS. 1 to will be described below.

One hundred parts by weight of a powdered polyethylene (passable througha -mesh screen) having a melt index of 4.0 and a specific gravity of0.922, l5 parts by weight of azodicarbonamide and 2 parts by weight ofzinc stearate are mixed in a ribbon blender for minutes at roomtemperature. The resulting mixture is put into the second chamber 53from the material throw-in opening 57 of the vacuum hopper 52 to abouthalf of its height. The air cylinder 56 is then actuated to close thesecond chamber 53 by means of the valve 55. Further, the mixture is putinto the first chamber 53 to about half of its height. The throw-inopening 57 is then closed, and the pressures of the first chamber 53 andthe second chamber 54 are reduced by means of the suction pipes 58 and59 to 550 mmHg below the atmospheric pressure, whereby the air containedin the mixture is discharged. With the valve 55 being opened, themixture in the second chamber 54 is fed into the stock feed inlet 51 ofthe extruder 50. When there is no mixture in the first chamber 53, thevalve 55 is closed and the mixture is put into the first chamber 53. Thepressure of the first chamber 53 is then reduced to 550 mmHg below theatmospheric pressure. The mixture is kneaded and melted in the extruderat a temperature of about 135 C., and extruded into a flat sheet formfrom a T-die 60. The sheet is taken up by the take-up device 61consisting of a pair of pinch rolls to obtain a flat sheet S-lcontaining the foaming agent and having a thickness of 2.2 mm and awidth of 400 mm, which is then wound up onto the wind-up device 62.

The obtained flat sheet S-1 is mounted on the delivery device 69 shownin FIG. 2, and exposed to the irradiation of a total dose of 3 Mrad ofaccelerated electrons both at the top and bottom surfaces thereof bymeans of the accelerated electron generators 64 and 65 in theaccelerated electron irradiation chamber 63. This procedure gives across-linked flat sheet 5-2, which is then taken up by a take-up device67 consisting of a pair of pinch rolls and wound up with a wind-updevice 68.

The so obtained polyethylene flat sheet S-2 having a thickness of 2.2 mmand a width of 400 mm is passed through the aperture 4 while beingsupported by the supporting device 7 consisting of a pair of pinch rollsrotating at a surface speed of 2 meters/min. in the apparatus of FIG. 3,to cause it to fall vertically into the pre-heating chamber 2 maintainedat a temperature of about 200 C. The flat sheet 8-2 is pre-heated thereto about 160 C., and passed through the aperture 5 to cause it to fallvertically into the foaming chamber 3 maintained at a temperature ofabout 280 C. In the foaming chamber 3, the flat sheet S-2 is heated toabout 200 C. Since the polyethylene resin that constitutes the flatsheet 8-2 is cross-linked, the sheet is neither elongated excessively orto breakage by its own weight even when heated to a temperature in therange of about to 200 C. in the pre-heating chamber 2 and the foamingchamber 3. The temperature of the flat sheet S-2 heated to about 200 C.in the foaming chamber 3 is then accurately controlled in its widthwisedirection by the infrared heaters 9 disposed in the foaming chamber 3.By this accurate temperature control by the infrared heaters 9, astarting position of foaming of the flat sheet 5-2 is substantiallyconstant on a line along the widthwise direction thereof and hardlyfluctuates upwards or downwards. The flat sheet 5-2 is thus rapidly andsharply expanded three-dimensionally to about 30 times on line A alongthe widthwise direction thereof. At this time, wavy wrinkles W occur inthe widthwise direction in the flat sheet S-3 'owing to the rapidexpansion. These wrinkles hardly change in their wavy configurationsince the starting position of foaming does not fluctuate upwards ordownwards as mentioned above. Thus, the subsequent extending operationwill be very easily carried out.

The foamed flat sheet S-3 is passed through the aperture 6 from thefoaming chamber 3, and stretched in the widthwise direction by means ofthe extending device 14 by a ratio corresponding to expansion of theflat sheet 8-2, that is, to a final width of 1,200 mm. Consequently, thewrinkles occurring in the foaming operation are completely removed.

The extending of the foamed flat sheet 8-3 by the tentering device 14will be described in detail with reference to FIGS. 4 and 5.

The distance between the extending devices 14 and 14' is adjusted to onecorresponding to a ratio of widthwise direction of the flat sheet 8-2,that is, 1,200 mm in this case. Both sides of the vertically movingfoamed flat sheet S3 are inserted into the guide grooves 18 and 18' ofthe suction devices 17 and 17'. When the air inside the pressurereducing chambers 20 and 20' is discharged from the exhaust pipes 21 and21' 'by means of a vacuum pump, the outer air passes through a gapbetween the inner wall surfaces of the guide grooves 18 and 18' and bothsurfaces side of the foamed flat sheet S-3, further through the suctionholes 19 and 19' at the bottom surfaces of the guide grooves 18 and 18',and sucked into the pressure reducing chambers 20 and 20'. As a result,the sheet guide grooves 18 and 18 are maintained at reduced pressure,and both side surfaces of the foamed flat sheet S-3 are sucked to themoving belts 22 and 22 sliding in intimate contact with the bottomsurfaces of the guide grooves 18 and 18. In this state, the flat sheet8-3 is transferred in the moving direction of the belts 22 and 22. Themoving speeds of the moving belts 22 and 22' are made the same as thespeed of pulling the flat sheet S-3. The flat sheet 8-3 is then cooledby the cooling rolls l5, and taken up by a take-up device 16 consistingof a pair of pinch rolls rotating at a surface speed of 8 meters/min.Since the flat sheet S-3 has been considerably cooled by the outer airafter leaving the foaming chamber 3, the surface of the flat sheet S3 isnot marred when contacted with the surfaces of the cooling rolls l5.

The so obtained foamed flat sheet S-3, mm thick and 1,200 mm wide, has auniform and fine cellular structure with a beautiful, smooth surface inwhich a ratio of expansion is about 30, and the cells have a diameter ofabout 0.1 mm. Also, the obtained foamed flat sheet has a uniformthickness and is free from wrinkles.

As another preferred embodiment, the foaming of a tubular sheet by meansof the apparatus of FIG. 6 will be described below.

In FIG. 6, the reference numeral 25 indicates a-preheating chamber, and26, a foaming chamber connected to the pre-heating chamber 25. Both thepreheating chamber 25 and the foaming chamber 26 are constructed ofheat-insulating walls. In the upper wall of the pre-heating chamber 25is provided an aperture 27 for passage of a tubular sheet in a flattenedform. Annular openings 28 and 29 for passage of the tubular sheet areprovided on a boundary wall between the preheating chamber 25 and thefoaming chamber 26 and on a lower wall of the foaming chamber 26. Abovethe aperture 27 of the pre-heating chamber 25 is provided a supportingdevice 30 for the flattened tubular sheet. This supporting device 30consists of a pair of pinch rolls so that the sheet is transferred tothe pre-heating chamber 25 while being supported by this pair of pinchrolls. The pre-heating chamber 25 includes three annular infraredheaters 31, and the foaming chamber 26, three annular infrared heaters32. These heaters are provided at predetermined intervals and in amanner surrounding the tubular sheet. I-lot air inlets 33 and 34 areprovided respectively at the bottom parts of the pre-heating chamber 25and the foaming chamber 26, and hot air outlets 35 and 36 are providedat the upper parts of the pre-heating chamber 25 and the foaming chamber26, respectively. Hot air introduced into the pre-heating chamber 25vand the foaming chamber 26 from the hot air inlets 33 and 34respectively rises along the surface of the tubular sheet, and isexhausted from the hot air outlet 35 and 36.

By partitioning the pre-heating chamber 25 from the foaming chamber 26with a boundary wall, the temperature of the pre-heating chamber 25 andthe foaming chamber 26 can be controlled independently.

An extending device for a tubular sheet is shown at 37, which consistsof a mandrel 30with a cylindrical portion having a diametercorresponding to a ratio of the widthwise expansion of the foamedtubular sheet, i.e., 680 mm, a gas-inlet pipe 39 piercing approximatelythrough the center of the said mandrel 30, a flattening device 40, acutter 41, and a mandrel supporting means 42. The reference numeral 43represents an inlet opening for the gas-inlet pipe 39, and 44, itsoutlet opening. A cooling jacket 45 isdisposed inside the cylindricalportion of the mandrel 38. The reference numeral 46 indicates a take-updevice consisting of a pair of pinch rolls, 47 and 48, guide rolls, and49, a wind-up device.

The T-die in the apparatus shown in FIG. 1 is replaced with a circulardie. A mixture consisting of 100 parts by weight of a powderedpolyethylene (passable through a -mesh screen) having a melt index of7.0 and a specific gravity of 0.920, 13 parts by weight ofazodicarbonamide, 0.7 part by weight of zinc stearate and 0.3 part byweight of zinc oxide is put into an extruder after having removed airfrom the mixture by reducing the pressure within the vacuum hopper shownin FIG. 1 to a pressure 450 mmHg below the atmospheric pressure. Themixture is kneaded and melted at C. and extruded into a tubularform bythe inflation method to thereby form a tubular sheet 5-1 I having athickness of 1.1 mm and a width in a flattened state of 330 mm. Thesheet is subjected in the flattened state to the irradiation ofaccelerated electrons at both the top and bottom surfaces thereof bymeans of the apparatus shown in FIG. 2 to form a crosslinked tubularsheet 5-22. The total dose is 2.4 Mrad.

The so obtained cross-linked polyethylene tubular sheet S-22 having athickness of 1.1 mm and width in the flattened state of 330 mm is passedthrough the aperture 27 while being supported by the supporting device30 consisting of a pair of pinch rolls rotating at a surface speed of 1meter per minute, and is caused to fall vertically into the pre-heatingchamber 25. The tubular sheet 8-22 is heated to about C. by the hot airand the infrared heaters 31, passed through the annular opening 28, andcaused to fall vertically into the foaming chamber 26. A gas such as airto be sent from the inlet 43 to the outlet 44 of the gas-inlet pipe 39is introduced into the tubular sheet 8-22 at a pressure of about 10 mmaq. and the flattened tubular sheet 8-22 is maintained in a tubularform. The tubular sheet 8-22 is heated by the hot air to about 200 C.,and the temperature of the tubular sheet 5-22 in its widthwise directionis accurately controlled by the infrared heaters 32. When the heating iscarried out in this way, the starting position of foaming of the tubularsheet 8-22 is substantially set along the widthwise direction of thetubular sheet 5-22 and does not fluctuate upwards or downwards. Thus,the tubular sheet 5-22 is expanded rapidly and sharply to about 25 timesthreedimensionally along the line A in its widthwise direction. Wavywrinkles W occur at this time in the foamed tubular sheet 8-33 owing tothe rapid expansion in the widthwise direction. These widthwise wavywrinkles are however removed completely, as the foamed tubular sheet8-33 is immediately stretched widthwise by a ratio corresponding to aratio of widthwise expansion of the tubular sheet 8-22 by the pressureof a gas such as air introduced into the tubular sheet S-22.

The foamed tubular sheet S-33 completely free from the wrinkles is putover the mandrel 38 and pulled downwards with a very thin layer of gassuch as air between the inner surface of the foamed tubular sheet andthe surface of the cylindrical portion of the mandrel 38. While passingthe mandrel 38, the foamed tubular sheet 8-33 is cooled. The foamedtubular sheet S-33 is then flattened by the flattening device 40, andboth ends of the foamed tubular sheet are continuously cut in thelongitudinal direction with the cutter 41 to thereby give two foamedsheets. The sheets are passed through a guide roll 47, taken up by atakeup device 46 consisting of a pair of pinch rolls rotating at asurface speed of 3.3 meters per minute, and wound up onto a wind-updevice 49 via a guide roll 48.

Thus, the use of the pressure of a gas such as air and a mandrel resultsin an easy removal of wavy wrinkles of the foamed tubular sheet 5-33which have occurred in the widthwise direction. In addition, thewidthwise size of the foamed tubular sheet 8-33 is well regulated by theaction of the mandrel. Since a part of a gas such as air introduced intoabout the center of the mandrel passes between the inner surface of thefoamed tubular sheet S33 and the surface of the cylindrical portion ofthe mandrel and spontaneously flows out into the outer atmosphere frombetween the longitudinally cut foamed sheets, the inside surface of thefoamed tubular sheet 8-33 does not directly come into contact with thesurface of the mandrel, and therefore is not marred at all.

The so obtained two foamed sheets, 2.4 mm thick and 1,050 mm wide, havea uniform and fine cellular structure with a beautiful, smooth surfacein which a ratio of expansion is about 25 and the cells have a diameterof about 0.2 mm. Also, the sheets have a uniform thickness and are freefrom wrinkles.

While the invention has been described in connection with the preferredembodiments, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from the spiritand scope of the invention.

We claim:

1. A process for the production of a foamed sheet of an ethylenic resin,which comprises continuously passing downwardly a long strip of asheet-like molded article of a cross-linked ethylenic resin, selectedfrom polyethylene, an ethylene/vinyl acetate copolymer, anethylene/propylene copolymer, an ethylene/butadiene copolymer, anethylene/butene-l copolymer and an ethylene/vinyl chloride copolymer,containing a normally solid organic blowing agent capable of evolvinggases upon decomposition by heating; preheating said sheet-like moldedarticle in transit to a temperature below the decomposition temperatureof said organic blowing agent; thereafter, while in transit, rapidlyheating said preheated sheet-like molded article to a temperature abovethe softening temperature of the crosslinked ethylenic resin and abovethe decomposition temperature of said organic blowing agent to therebyfoam said sheet-like molded article, whereby the starting position offoaming along the widthwise direction of said sheet-like articlefluctuates minimally; and extending said foamed sheet-like article inthe widthwise direction to an extent as to thereby remove wavy wrinklesoccurring in the widthwise direction of said article at the time offoaming.

2. The process of claim 1 wherein said sheet-like molded article is aflat sheet, and the widthwise extending of said foamed flat sheet iseffected by inserting both sides of said foamed flat sheet into twoopposing guide grooves spaced from each other by a distancecorresponding to a width of the expanded flat sheet in its widthwisedirection, said guide grooves each having suction holes at the bottomand a moving belt at the bottom, which belt has air-permeable apertures,sucking air inside the guide grooves from said suction holes, andtransferring the foamed flat sheet while both side of the flat sheet arecaused to stick to the moving belt by the suction force.

3. The process of claim 1 wherein said sheet-like molded article is atubular sheet, and the widthwise extending of said foamed tubular sheetis effected by putting a tubular sheet foamed after passage through asupporting means consisting of a pair of pinch rolls oyer a mandrelhaving a cylindrical ortion with a diameter corresponding to a width 0the expanded sheet in the widthwise direction, passing it along thecylindrical portion of the mandrel while introducing a gas into thefoamed sheet situated between said supporting means and said mandrel viaa hole piercing through said mandrel, and transferring the sheet whilecutting it along its longitudinal direction.

4. The process of claim 1 wherein said ethylenic resin is a low-densitypolyethylene and said organic blowing agent is azodicarbonamide.

5. The process of claim 1 wherein said sheet-like molded article isproduced by kneading and extruding a mixture of said ethylenic resin, anorganic blowing agent having a decomposition temperature higher than themelting temperature of said resin and an organic peroxide having adecomposition temperature lower than the decomposition temperature ofsaid organic blowing agent at a temperature at which said organicperoxide is not substantially decomposed and while said mixture is in astate substantially free from air inside; and heating the resultingsheet-like molded article to a temperature at which said organicperoxide is substantially decomposed but at which said organic blowingagent is not substantially decomposed to thereby cross-link saidethylenic resin constituting said sheetlike molded article.

6. The process of claim 5 wherein said ethylenic resin is a low-densitypolyethylene, said organic blowing agent is azodicarbonamide, and saidorganic peroxide is dicumyl peroxide.

7. The process of claim 1 wherein said sheet-like molded article isproduced by kneading and extruding a mixture of said ethylenic resin andan organic blowing agent having a decomposition temperature higher thanthe melting temperature of said ethylenic resin at a temperature atwhich said organic blowing agent is substantially not decomposed andwhile said mixture is in a state substantially free from air inside; andsubjecting the molded sheet-like article to ionizing radiation tothereby cross-link said ethylenic resin constituting said sheet-likearticle.

8. The process of claim 7 wherein said ethylenic resin is a powderedlow-density polyethylene, said organic blowing agent isazodicarbonamide, and metal salts of fatty acids are further added.

9. The process of claim 7 wherein a screw extruder provided with avacuum hopper at its material feed inlet is used, and after putting themixture into the vacuum hopper to suck and remove air inside themixture, the mixture is kneaded and melted in the extruder and thenextruded.

10. A process of claim 7 wherein a screw extruder provided with a ventopening is used, and while sucking and removing air inside the mixturefrom said vent opening, the mixture is kneaded and melted in theextruder and then extruded.

2. The process of claim 1 wherein said sheet-like molded article is aflat sheet, and the widthwise extending of said foamed flat sheet iseffected by inserting both sides of said foamed flat sheet into twoopposing guide grooves spaced from each other by a distancecorresponding to a width of the expanded flat sheet in its widthwisedirection, said guide grooves each having suction holes at the bottomand a moving belt at the bottom, which belt has air-permeable apertures,sucking air inside the guide grooves from said suction holes, andtransferring the foamed flat sheet while both side of the flat sheet arecaused to stick to the moving belt by the suction force.
 3. The processof claim 1 wherein said sheet-like molded article is a tubular sheet,and the widthwise extending of said foamed tubular sheet is effected byputting a tubular sheet foamed after passage through a supporting meansconsisting of a pair of pinch rolls over a mandrel having a cylindricalportion with a diameter corresponding to a width of the expanded sheetin the widthwise direction, passing it along the cylindrical portion ofthe mandrel while introducing a gas into the foamed sheet situatedbetween said supporting means and said mandrel via a hole piercingthrough said mandrel, and transferring the sheet while cutting it alongits longitudinal direction.
 4. The process of claim 1 wherein saidethylenic resin is a low-density polyethylene and said organic blowingagent is azodicarbonamide.
 5. The process of claim 1 wherein saidsheet-like molded article is produced by kneading and extruding amixture of said ethylenic resin, an organic blowing agent having adecomposition temperature higher than the melting temperature of saidresin and an organic peroxide having a decomposition temperature lowerthan the decomposition temperature of said organic blowing agent at atemperature at which said organic peroxide is not substantiallydecomposed and while said mixture is in a state substantially free fromair inside; and heating the resulting sheet-like molded article to atemperature at which said organic peroxide is substantially decomposedbut at which said organic blowing agent is not substantially decomposedto thereby cross-link said ethylenic resin constituting said sheet-likemolded article.
 6. The process of claim 5 wherein said ethylenic resinis a low-density polyethylene, said organic blowing agent isazodicarbonamide, and said organic peroxide is dicumyl peroxide.
 7. Theprocess of claim 1 wherein said sHeet-like molded article is produced bykneading and extruding a mixture of said ethylenic resin and an organicblowing agent having a decomposition temperature higher than the meltingtemperature of said ethylenic resin at a temperature at which saidorganic blowing agent is substantially not decomposed and while saidmixture is in a state substantially free from air inside; and subjectingthe molded sheet-like article to ionizing radiation to therebycross-link said ethylenic resin constituting said sheet-like article. 8.The process of claim 7 wherein said ethylenic resin is a powderedlow-density polyethylene, said organic blowing agent isazodicarbonamide, and metal salts of fatty acids are further added. 9.The process of claim 7 wherein a screw extruder provided with a vacuumhopper at its material feed inlet is used, and after putting the mixtureinto the vacuum hopper to suck and remove air inside the mixture, themixture is kneaded and melted in the extruder and then extruded.
 10. Aprocess of claim 7 wherein a screw extruder provided with a vent openingis used, and while sucking and removing air inside the mixture from saidvent opening, the mixture is kneaded and melted in the extruder and thenextruded.